The zeolite known under the name mazzite was discovered in 1972 (GALLI et al. Contr. Mineral. and Petrol 45 99 (1974)) in the basalts of Mount Semiol, Loire, France. The structure of the mazzite (Rinaldi et al. Acta Cryst. B31 1603 (1975)) is described by an assembly of six columns of gmelinite cages, which corresponds to two hexagonal faces, 9 square faces and three octogonal faces joined together with a displacement of .+-.1/4.
The three-dimensional lattice is characterized by a system of three channels which are parallel to the crystallographic axis c:
a--quasi-cylindrical channels bordered by 12-tetrahedral rings whose free diameter is between 0.75 and 0.8 nm. The walls of these channels are lined with ladders consisting of alternating rings made up of 4 tetrahedrons of gmelinite cages and of rings made up of 5 tetrahedrons, which are the result of joining the columns;
b--secondary channels, located between the columns of gmelinite cages and consisting of rings made up of 8 tetrahedrons forming a highly distorted chair conformation, and
c--channels inside the columns made up of cages, accessible via the rings consisting of 6 tetrahedrons.
The zeolites of the mazzite type are characterized by a unique X-ray diffraction pattern.
The best-known and best-characterized examples of this type of zeolite are the zeolites omega and ZSM-4. They are solids which are of interest for adsorption and catalysis.
The synthesis of the zeolite omega is described in U.S. Pat. No. 4,241,036 (UNION CARBIDE) and U.S. Pat. No. 4,091,007 (MOBIL OIL). French Patent 2,074,007 (MOBIL OIL) and British Patent 1,297,256 (MOBIL OIL) describe the synthesis of ZSM-4. A solid which is isostructural with zeolite omega and called LZ 202 is described in Patent Application PCT WO 87/00,158 (UNION CARBIDE).
The synthesis of the zeolites omega and ZSM-4 is likewise described in the scientific literature by Aiello et al. (J. Chem Soc. A 1970 1470), Dwyer et al. (J. Catal 59 263 (1979)), Cole et al. (Adv. Chem. Ser. 121 583 (1973)), Perrotta et al. (J. Catal. 55 240 (1978)), Araya et al. (Zeolites 4 263 (1984)), Fajula et al. (Zeolites 7 203 (1987) ) and Nicolas et al. (Stud. Surf. Sci. Catal. 37 115 (1987)).
Zeolite omega and zeolite ZSM-4 are prepared by hydrothermal crystallization of reactive alkaline gels of aluminosilicates. With the exception of the one described in Patent Application WO 87/00,158, all syntheses are carried out in the presence of an organic reagent, such as tetramethylammonium (TMA), pyrrolidine, choline, diazobicyclooctane or triethylenediamine. TMA is the most common and most selective organic reagent. These organic compounds are used as such or in the form of hydroxide or in salt form. The addition of inorganic bases is necessary to obtain the required level of alkalinity. For the synthesis of zeolite omega and zeolite ZSM-4, the inorganic base used is sodium hydroxide. The reaction medium can contain lithium but does not tolerate potassium or tolerates only a small amount of it (Aiello et al. J. Chem. Soc. A 1970 1470 and Cole et al. Adv. Chem. Ser. 121 583 (1973)). In the presence of potassium, offretite is in general obtained.
The general formula of the synthetic zeolites of the mazzite type in their form as synthesized can be written in terms of moles of oxides: ##EQU1## where m is an n-valent cation, in general sodium, and A is an organic reagent carrying m positive charges.
According to a particular feature of the aluminosilicate, such as the zeolites, the cations and the organic compounds incorporated in the course of the synthesis can be removed and replaced by other cations. These operations are even necessary for freeing the pores of the zeolite and generate catalytic activity.
The organic reagents are more conventionally removed by oxidative calcination. The cations M are exchangeable for other cations in aqueous medium by well-known methods. The most useful cations for catalysis are the proton or cations belonging to the class of rare earths.
However, these exchange treatments induce an embrittlement of the zeolitic skeleton, especially during the operations which require the application of elevated temperatures in the presence of water vapour. The phenomenon of embrittlement is well known; it is caused by reactions which hydrolyze the Al-O-T bonds, where T is a cation of the lattice, in general silicon, the reactions being catalysed by the protons which are present in the lattice to neutralize the excess negative charge associated with the presence of aluminium. In the course of prolonged hydrothermal treatments, the zeolite structure gradually decomposes and finally becomes amorphous.